1. Field of the Invention
The present invention relates to an optic column which can produce a micro beam (generally in 0.5 or less .mu.m) at large current in the low energy region for example of not more than 1 keV, and particularly to an optic column which can produce a micro beam at large current in a simple structure.
2. Related Background Art
An electron optic system for obtaining a micro beam at large current conventionally employs such a popular technique that a beam from a circular hot cathode (point source of light) is focused by a demagnifying lens system into a circular beam. The electron optic system is constituted by a lens of rotation symmetry system. In this method, the upper limit of obtained current is restricted by the current density of electrons emitted from the point source and the space-charge effect, because the electrons are drawn from a small area at the tip of cathode which is the point source.
In order to eliminate this restriction, an attempt is made to increase an area contributing to the electron emission as much as possible by increasing the diameter of the cathode tip in electron gun or by face-processing the tip portion of cathode. However, even if the diameter of the tip end of cathode is increased, the maximum current is basically limited by the current density of electrons emitted from cathode and the space-charge effect near the tip end of cathode in case of the electron optic column using the lens constitution of rotation symmetry system.
To solve the problem, there is a method considered to increase the probe current by using a line cathode, which increases the area of electron emission and relieves the restriction of emission current forced by the space-charge effect. The current is expected to increase in this arrangement as the emission area increases, because the space charge basically affects the current density.
The line cathode was proposed by Brodie and Nixon (J. Vac. Sci. Technol. B7 (6), November/December, p1878, 1989). They conducted theoretical analysis while observing the spread of emission energy caused by the Boersch effect in the space-charge effect of line cathode. They found that 27% of energy spread was alleviated as compared with the conventional point (circular) cathode. They explained it as the alleviation of space-charge effect by means of the line cathode. There is, however, nothing described in the document about the increase effect of maximum emission current amount itself with increase of emission area.
The alleviation due to the Boersch effect decreases the energy spread, whereby the beam diameter is prevented from being widened by the chromatic aberration of lens system. This results in improving the performance of focusing lens. The chromatic aberration increases its influence in the low energy region, and therefore this method is very effective to obtain a micro beam at large current and in low energy.
Nixon (U.S. Pat. No. 4,804,851) proposed an idea to form a line image of a line cathode. A beam is shaped by an aperture or slit (called as blocking part) to change the length (aspect ratio) of the line image.
The proposal of Nixon concerns only the formation of line image from the line cathode, which is not such a method that a micro beam is formed by beam-shaping with adjustment of excitation condition of asymmetric optic system such as a multi-pole lens.
Brodie (J. Vac. Sci. Technol. B8, p1691, 1990) proposed the method to form a line beam with variable aspect ratio (a ratio between length and width for rectangular beam) for electron beam lithography by using a doublet (device for shaping a beam by two steps of multi-pole lenses) or a triplet (device for shaping a beam by three steps of multi-pole lenses) of electric field type quadrupole lenses and the line cathode. The stigmatic focus condition is a lens condition that two-directional orbits, i.e., X orbit (major-axis orbit) and Y orbit (minor-axis orbit), are coincident with each other on image plane on the center axis (Z-axis) in multi-pole lens and that an X-directional magnification Mx is equal to a Y-directional magnification My. On the other hand, the pseudo stigmatic focus condition is a lens condition that the X-directional orbit and Y-directional orbit are coincident with each other on the image plane on the Z-axis and that Mx and My are different from each other.
Brodie describes the construction of electron optic system to obtain a line beam in aspect ratio of approximately 1:1 to 1:100 by combinations of line cathode light source with quadrupole lenses, and particularly the operation conditions of quadrupole lenses. The electron optic system of Brodie comprises, as shown in FIG. 6, a line cathode light source 11, two steps of quadrupole lenses 12 for shaping emitted electrons into a beam of arbitrary aspect ratio, two steps of magnetic field type condenser lenses 13 for demagnifying the beam shaped in rectangle to obtain a micro beam, and a magnetic field type objective lens 14.
According to the operation conditions of quadrupole lenses 12 proposed by Brodie, the excitation condition is low, so that the demagnification ratio (inverse of magnification) is low, and the magnification ratio Mx/My is also as low as about 1/10. Thus, the quadrupole lenses 12 cannot produce a micro beam, and therefore the quadrupole lenses 12 are used as lenses for probe shaping as a result. Then, the condenser lens 13 and the objective lens 14 are added as demagnifying lens system to enhance the demagnification ratio of the entire system. This results in making the electron optic system complex. In particular, the example of FIG. 6 is so arranged that the condenser lens 13 and the objective lens 14 are formed as magnetic field type lenses, which makes the entire system larger and heavier.
As for the technique for shaping and focusing a beam by means of multi-pole lens system, Okayama and Tsurushima (Japanese Laid-open Patent Application No. 60-233814 and Japanese Patent Publication No. 2-49533) proposed an idea and the optimum conditions, in which a line image is formed by quadrupole lens system from a point cathode and in which its length (aspect ratio) and current density distribution are controlled by changing the excitation condition of quadrupole lens system. Two steps of quadrupole lenses are shown as the quadrupole lens system in an embodiment, though they describe that the same effect can be achieved by a combination of more than two multi-pole lenses. However, a beam is not focused only by the multi-pole lens system either, as in Brodie. The multi-pole lens system functions only to shape the beam, and the final focusing is carried out by a magnetic field type objective lens in the conventional manner. These are applications of multi-pole lens system to obtain a line beam from a point cathode, but are different from the method for shaping and focusing a beam from line cathode into a point beam.
As described above, the method for focusing a beam from circular point source of light into a circular beam has a limit for current of obtainable electron beam due to the emission current density of electrons from cathode and the space-charge effect. On the other hand, the arrangement in combination of line cathode with quadrupole lenses, as shown in FIG. 6, can greatly relax the restriction due to the emission current density from cathode and the space-charge effect, but has a low demagnification ratio of quadrupole lens system, which makes it difficult forming a micro beam by quadrupole lens system alone and which requires an additional demagnifying lens system. This results in making the electron optic system complex and increasing the size of apparatus.